The present invention is related to a communication terminal comprising a transmitter for transmitting bursts of a modulated carrier, the terminal comprises an audio signal processing arrangement.
Such communication terminal is known from U.S. Pat. No. 4,932,061.
In mobile radio systems often use is made of time domain multiple access (TDMA) in order to enable different communication to access the same radio channel. In TDMA the radio channel is subdivided into a number of subsequent time slots. Each of the timeslots can be assigned to a communication terminal for communication with the base station. For doing so, the communication terminal comprises a transmitter for transmitting burst of data modulated on a carrier in the time slot assigned to it. In general the assigned time slot has a fixed position in a transmission frame, resulting in a periodic transmission of the information burst.
Due to the periodic nature of the burst and the fast changing signal at the beginning and the end of the burst it can easily happen that the RF bursts emitted by the communication terminal causes interference in the audio part of the communication terminal. This can cause transmission of a periodic noise like signal to the base station, and to the presence of a similar noise like signal in the audio signal received from the base station.
Although it it possible to reduce the amount of interference by proper printed circuit board lay out, it is experienced that reduction of the interference to an inaudible level leads to a drastic increase of the costs involved with manufacturing the communication terminal. Furthermore it leads to a more bulky design of the communication terminal, which is of course highly undesirable.
The object of the present invention is to provide a communication terminal according to the preamble, in which the manufacturing costs involved with the suppression of the interference signal due to the emission of the bursts is substantially reduced.
To achieve said object, the communication terminal according to the invention is characterized in that that the audio signal processing arrangement comprises interference suppressing means for suppression a known interference signal being caused by the transmission of the bursts of the modulated signal.
By using interference suppressing means in the audio processing means, it becomes possible to reduce the interference signal to an inaudible amount without needing expensive measures. Because the basic properties of the interference signals, such as the spectral composition of the interfering signal, are known in advance, the complexity of the interference suppressing means can be substantially reduced.
An embodiment of the present invention is characterized in that the interfering suppression means comprise frequency separating means for separating its input signal into a plurality of sub-band signals, amplitude adjusting means for adjusting at least the amplitude of, the sub-band signals comprising spectral components of the interference signal, and combining means for combining the sub-band signals into a combined output signal.
An easy and effective way of suppressing the interference signal is the use of noise suppressing techniques, in which the input signal is split into a number of sub-bands. The amplitude of the signal in the sub-bands containing interference signals are reduced. Subsequently, the sub-band signals are combined again into a signal in which the interference signal is strongly reduced.
A further embodiment of the present invention is characterized in that the amplitude adjusting means are arranged for multiplying the amplitude of the sub-band signals comprising spectral components of the known interference signal with a value which decreases with increasing strength of the component of the known interference signal.
By reducing the amplitude of the sub-band signals in dependence on the strength of the interference, it is obtained that the changes made to the audio signal are not larger than is needed to suppress the interference signal.